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Deck calculator

A deck materials list is area math plus spacing math. Boards cover the rectangle; joists march at 12 / 16 / 24 in on center; waste covers rips and bad ends. This page is for shopping lumber. It is not a beam/footing design and it does not stamp posts for snow load.

ft
ft
in
in
in O.C.
10%
0510152025

+10% so you don't come up short

ORDER ESTIMATEDECK
Boards to buy29
Joists13
Area192 sq ft
Waste10%
How it calculates
boards across = ceil(width × 12 ÷ (face + gap)); order = ceil(across × (1 + 10%)); joists ≈ floor(length × 12 ÷ spacing) + 1.

Estimate only — not a contractor bid. Confirm bag yields and local codes before you pour or order.

When to use

Rectangular deck shopping list; comparing composite board widths; rough joist counts before a lumber run.

When not to

Multi-level decks with landings as one number — split into rectangles. Engineered beam sizing — use span tables / engineer. Curved edges — add waste by judgment.

Assumptions

Board laying along width or length is your choice (affects piece lengths). Gap between boards editable. Joist spacing default 16 in O.C. Waste default 10%. Ledger/beam hardware not counted.

Worked examples

Example 1

12×16 ft deck, 5.5 in boards, 1/8 in gap, waste 10%
→ board count from 12 ft side coverage × 1.1 waste

Picture-frame borders add linear feet not in the simple field.

Example 2

same deck, joists 16 in O.C. across 12 ft span direction along 16 ft
→ joist count ≈ 13 (including ends) — confirm ledger detailing separately

How it calculates

Cover the deck width with board face + gap, add waste, then count joists along the length.

  1. Enter deck length and width.
  2. Set board face width, gap, and joist spacing.
  3. Dial waste; read board count, joist count, and area.
area = L × W
boards ≈ ceil(cover_direction ÷ (board_width + gap)) × (1 + waste%)
joists ≈ floor(span ÷ spacing) + 1

Common traps

  1. Actual vs nominal — a “6 in” deck board is not 6.000 in face.
  2. Diagonal decking needs more waste than straight.
  3. Joist direction changes length stock you buy, not only count.
  4. Hidden fasteners change gap assumptions.

Next tool

Board-only focus → deck board calculator.

Stairs off the deck → stair calculator.

Concrete footings under posts → concrete calculator.

FAQ

Cover dimension ÷ (board width + gap), plus waste.

Related

Estimate only — not a contractor bid or engineered takeoff. Confirm bag yields and local codes before you pour or order.